(Education Week) From the early days of the Common Core State Standards, the two national teachers’ unions have been among the initiative’s biggest boosters, helping to make the case to the nation’s 3.5 million teachers for the tougher expectations and putting significant money into the development of aligned curricula and tools.

But in some union quarters, that support is starting to waver—the product of flawed implementation in states, concerns about the fast timeline for new testing tied to the standards, and, in at least one instance, fallout from internal state-union politics.

The unions’ evolving positions raise new questions about the standards’ durability at a time when the common core has been buffeted by criticism: from conservatives worried about a loss of state and local control, and from progressives fretting about the impact on teacher evaluations, classroom instruction, and student assessment.